Calibration and validity of an eccentric photorefractor.

A new graphical calibration method was developed to convert the photorefractive reflex into refractive error. With this graphical method, the refractive error can be obtained for pupil sizes and for photorefractive reflex sizes which have not been precalibrated. In the conventional method, the refractive error associated with non-precalibrated pupil and reflex sizes is obtained by interpolating between neighbouring precalibrated points. This introduces error because the relationship between refractive error and pupil and reflex size is not linear. Three hundred and sixty-one children aged between 36 and 65 months were clinically examined and photorefracted. The refractive error obtained using retinoscopy and the eccentric photorefractor agreed well with each other, although the photorefractor tended to under-estimate refractive error. Using the referral criteria developed by Chan, O.Y.C. and Edwards, L.M. (Refraction referral criteria for Hong Kong Chinese Children. Ophthal. Physiol. Opt. 14, 259-256, 1994), the photorefractor had a sensitivity of 74.6% and a specificity of 96.4%. All the under-referred cases were borderline, having just failed the referral criteria. All the cases with hyperopia of > + 2.0 D, astigmatism of > 1.25 D and visual acuity of worse than 6/18 were identified.